1. Field of the Invention
The present invention relates to a semiconductor device comprising a diode and a switching device made of silicon carbide.
2. Description of the Background Art
Diodes and switching devices such as thyristors and transistors have conventionally been used in many semiconductor devices.
When a switching device is in an ON-state or a diode is in a forward bias state, that is, a device is in a conductive state, a relatively large amount of charge accumulates inside the device. On the other hand, when the switching device is in an OFF-state or the diode is in a reverse bias state, that is, the device is in a nonconductive state, a space-charge layer is formed inside the device, without a large amount of charge existing therein.
Therefore, during a process in which the device undergoes a transition from a conductive state to nonconductive state (turn-off), the charge accumulating inside the device has to be vanished. During this process, current larger than that in the nonconductive state flows, which causes power dissipation (turn-off power dissipation). Larger the amount of charge accumulating in the device in the conductive state, larger current flows at the turn-off, which results in an increase in the turn-off power dissipation.
Moreover, during a process in which the device undergoes a transition from the nonconductive state to the conductive state, charge should accumulate inside the device. During this process, voltage necessary for flowing current through the device becomes higher than that in the conductive state, which causes power dissipation (turn-on power dissipation). Larger the amount of charge accumulating inside the device during this process, more time is required for the accumulation, which results in an increase in the turn-on power dissipation.
The sum of the turn-on power dissipation and the turn-off power dissipation is called switching loss. Especially in a high-voltage device, the switching loss is great, by which a semiconductor device is seriously affected. This imposes essential limitations on the switching performance of the device. As a result, limitations shall be imposed on the performance of the semiconductor device.
The foregoing shows that a reduction of the amount of charge accumulating in the conductive state of the device makes it possible to suppress the switching loss. However, when a small amount of charge accumulates in the conductive state, a voltage drop in the conductive state becomes greater, which means an increase in conduction loss of the device.
In other words, the switching loss and the conduction loss have the trade-off relationship. The relationship depends on the voltage blocking capability of the device and deteriorates as the voltage blocking capability increases.
Further, making the device thin allows a reduction of the conduction loss as well as allowing large current to flow with a small amount of charge. This improves the relationship between the switching loss and the conduction loss. However, making the device thin causes deterioration in the withstand voltage characteristics of the device. Therefore, for a device with high voltage such as a power converter, there is a limit in making the device thin.
For the sake of solving the above problems and achieving energy saving, positive attempts are being made to improve the relationship between the switching loss and the conduction loss in a diode and a switching device used for power conversion in a semiconductor device. In one of such attempts, a diode and a switching device which have conventionally been made of silicon are made of silicon carbide.
Since silicon carbide has a reverse breakdown field substantially ten times that of silicon and excellent withstand voltage characteristics, it is suitable for a device that operates under the condition where high voltage is generated in a blocking state of the device. In other words, a thickness of the device necessary for maintaining voltage of a value can be made much thinner than that of a device made of silicon necessary for maintaining voltage of the same value. This is expected to contribute to an improved relationship between the switching loss and the conduction loss.
Further, since silicon carbide has a wide energy gap between bands and an excellent thermal stability, a device made of silicon carbide can operate at high temperatures approximately under 1000 kelvin. Furthermore, since silicon carbide has a high thermal conductivity to thereby radiate heat effectively, a device made of silicon carbide can be arranged with high density. In view of these characteristics as well, silicon carbide is expected to be applied to a next-generation power semiconductor device.
The diode and switching device made of a semiconductor material such as silicon or silicon carbide as described above obtain the capability of blocking voltage by means of space-charge layers formed therein. On the other hand, charge accumulates in the stationary conductive state, which allows current to flow with low voltage.
More specifically, the diode and the switching device in the switching processes of turn-on and turn-off have capacitive components determined by voltage-current characteristics of the space-charge layers and discharge or absorption of charge. Further, there exist resistive components having, as parameters, a value of leakage current at the time of voltage blocking and that of current to be generated by movement of charge, resistive components determined by the state of accumulation of charge in an area outside the space-charge layers and an impurity concentration, and the like. Furthermore, a wiring for electrically connecting the device to another one is provided outside the diode and the switching device, and it includes an inductance component. Therefore, an LCR circuit is to be formed by the capacitive components, the resistive components and the inductance component in the semiconductor device comprising the diode and the switching device.
The concentration distribution of charge varies in the process of turn-on and turn-off of the diode and the switching device, so that the above capacitive components and the resistive components vary widely. With the variations, the condition of natural oscillation is easily satisfied in the LCR circuit, which causes voltage oscillation of the device. The oscillation might generate voltage of a value exceeding the voltage blocking capability of the diode and the switching device. The oscillation might also cause electromagnetic noise in peripheral equipment, which may contribute to blocking a normal operation of the equipment.
The oscillating amplitude of the voltage depends on a voltage applied to the semiconductor device and increases with an increase in the voltage applied. Further, when the resistive components are great and the LCR circuit has a great quality factor, the oscillating amplitude also increases.
As has been described, the device made of silicon carbide can be made much thinner than a device made of silicon for the purpose of improving the relationship between the switching loss and the conduction loss. However, in the case where the device made of silicon carbide is thin, there is a small amount of charge accumulating inside the device and being discharged from the device, so that the resistive components inside the device vary at a high speed in response to turn-on and turn-off. This means an abrupt increase in the resistive components especially at the time of turn-off. When the resistive components in the LCR circuit abruptly increase in the case where the condition of natural oscillation is satisfied and there is voltage oscillation in the LCR circuit, there occurs an increase in the quality factor which exceeds attenuation of voltage oscillation with time, and voltage oscillation having an extremely high amplitude is likely to occur.
Therefore, a diode and a switching device made of silicon carbide can be made thin to improve the relationship between the switching loss and the conduction loss, which, however, is likely to cause voltage oscillation of a device having a high amplitude. Thus, it is forced to limit a voltage to be applied to the device low in order to avoid breakage of the device and malfunction of peripheral equipment, which gives rise to deterioration in the performance of the semiconductor device.
It is an object of the present invention to provide a semiconductor device comprising a diode and a switching device made of silicon carbide that is capable of suppressing occurrence of voltage oscillation having a high amplitude, while improving the relationship between the switching loss and the conduction loss.
According to the present invention, a semiconductor device comprises at least one of a diode and a switching device each made of silicon carbide, wherein at least one of a resistor and a capacitor is connected in parallel to at least one of the diode and the switching device.
Therefore, it is possible to suppress variations in the resistive component and/or capacitive component accompanied with turn-on or turn-off of at least one of the diode and the switching device in the LCR circuit formed by an external wiring and at least one of the diode and the switching device.
Thus, the LCR circuit is unlikely to satisfy the condition of natural oscillation at the turn-on or turn-off of at least one of the diode and the switching device, which can suppress occurrence of voltage oscillation of the device.
Further, an increase in the quality factor of the LCR circuit can be suppressed at the turn-off of at least one of the diode and the switching device. Therefore, when voltage oscillation of the device occurs, an increase in the amplitude can be suppressed.
Therefore, it is possible to suppress occurrence of voltage oscillation having an extremely high amplitude also when a diode and a switching device made of silicon carbide is made thin to improve the relationship between the switching loss and the conduction loss. This allows avoidance of breakage of the device and malfunction of peripheral equipment. Further, this contributes to improved performance of the semiconductor device because there is no need to limit voltage applied to the device low.
Preferably, at least one of the resistor and the capacitor is made of silicon carbide.
Therefore, the semiconductor device of the present invention is capable of operating stably under a high temperature condition.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.